Currently there will be an estimated 170,000 new cases of lung cancer diagnosed annually, and lung cancer will remain the leading cause of cancer deaths . The present treatment of locally advanced and metastatic lung cancer is very unsatisfactory, and the overall.5-year survival of patients diagnosed with lung cancer is only about 15%. The disappointing results of recent studies have led to the realization that we have reached a "chemotherapy efficacy plateau". A recent analysis of trials performed over the last 30 years also clearly demonstrated that only minimal progress has been made in the treatment of this disease. It has recently been shown that activating mutations in the Epidermal Growth Factor Receptor (EGFR) tyrosine kinase domain determine responsiveness to the therapies utilizing EGFR tyrosine kinase inhibitors (TKIs), gefitinib and erlotinib, in Non-small cell lung cancer (NSCLC) patients. Despite the dramatic responses to the TKIs in certain subsets of NSCLC patients, resistance to the TKIs does emerge over time primarily due to a T790M secondary mutation in the tyrosine kinase domain. Whereas the T790M mutation likely accounts for half the tumors with acquired resistance to TKIs, other mechanism(s) by which resistance to the TKIs emerges are not known. In addition, although we have showed that an alternative, irreversible anilinoquinazoline EGFR inhibitor, CL-387,785, appears to overcome the resistance, new strategies of treatment are sorely needed, considering that novel resistant mutations against the irreversible EGFR inhibitors should occur in the long run and most of NSCLC patients are not curable unless the tumors are detected in an early-stage. Thus, our goals in this proposal are: 1) to clarify the mechanisms for the growth advantage of the T790M secondary mutation 2) to identify novel EGFR mutations in order to elucidate mechanisms of tyrosine kinase inhibitor resistance 3) to develop a novel therapy which could apply to NSCLC patients who do not respond to TKIs. These studies will lead to new insights into mutant EGFR-driven tumorgenesis, as well as the identification of novel target pathways for therapeutic intervention in the future.